In machines which are used to manufacture and inspect semiconductor components, there is often the requirement to position objects precisely. For instance, it may be necessary for a wafer to be positioned extremely precisely under a tool of an exposure or inspection unit. In that case, the wafer lies on a table that is movable in six degrees of freedom and is moved via one or more suitable drives. Thus, the table acts as the object whose position must be detected extremely precisely. In order to position this table via an associated control unit, the generating of position signals with respect to the spatial position of the table by highly accurate position-measuring devices is necessary.
In particular, optical position-measuring devices such as interferometers, grating-based optical position-measuring devices or combinations of such devices are suitable for this purpose.
In order to accurately determine the spatial position of the object in several degrees of freedom using interferometers, U.S. Pat. No. 6,020,964, for example, describes probing the object optically from at least two sides. Disposed on two sides of the object or table are reflectors, upon which several measurement beams of associated interferometer measuring arms fall. What are termed multi-axis interferometers having several measurement beams or measuring arms are used per side, in order to also be able to detect rotational movements of the object about the various directions of movement. The at least two probing directions are usually oriented orthogonally relative to each other, so that the movement of the object is thus determinable in the plane which is defined by the two directions of movement. Possible movements of the object in a direction of movement perpendicular thereto may be detected via suitably diverted measurement beams which are reflected back again at reflectors above the object. To that end, suitable reflecting mirrors must be disposed on the object. A problem with this approach, however, is that the object must be probed optically via at least two sides. Consequently, these two sides must be as freely accessible as possible for the measurement beams. However, this cannot be ensured constructionally in all applications. In addition, the reflecting mirror on the object necessary for the diverting first of all increases the moving mass of the object, and secondly, can only be produced with considerable expenditure in the accuracy required.
German Published Patent Application No. 10 2012 201 393 describes detection of the position of an object in several spatial degrees of freedom, in which optical probing of the object is provided from only one direction. The limitations discussed above having several necessary probing directions are therefore inapplicable. However, a relatively complex offsetting of the position signals of various measurements is necessary in order to determine the movement of the object in the total of six degrees of freedom.